The invention relates in general to gas sensors and in particular to a gas-sensitive field-effect transistor with an air gap.
The functional principle of gas sensors based on field-effect transistors is known. When compared with other gas sensor technologies, this principle offers a number of advantages, in particular, a wide field of application as well as relatively low production and operating costs. The embodiments of the transistor known as suspended gate field effect transistor (SGFET) and capacitively controlled field effect transistor (CCFET) are especially advantageous. They are characterized by an air gap between a gate electrode and a transistor structure. The gas to be detected is diffused through the air gap and absorbed on a gas-sensitive layer. As a result, a change in potential is generated in the gas-sensitive layer which induces a change in the drain to source current within the transistor structure.
Precise adjustment of the sensitive layer in relation to the transistor structure is a desired parameter in the gas sensor. Accurate control of the air gap height is desirable since the coupling capacity of the gas sensor is directly correlated with the strength the of electrical signal indicative of the amount of detected gas provided by the gas sensor. Since the height of the air gap between the gate electrode and the channel region determines to a large extent the coupling capacity of the potential change induced by the gas absorption, particular attention is typically devoted in the fabrication of the gas sensor to the relatively precise formation of the air gap. Such precise adjustment of the air gap enables the reproducibility of the response characteristic and the sensitivity of the gas sensor, and also improves the bandwidth of the to-be-detected gas concentrations of the sensor.
The thickness of the sensitive layer employed is generally based on the type of gas-sensitive material and the gas to be detected. The desired layer thickness may differ for different layer materials, and care must be taken that this factor is incorporated as necessary into the dimensioning of the air gap.
A suitable method for producing various layer thicknesses is described, for example, in German patent DE19956744. This prior art patent discloses a structure using hybrid flip-chip technology having a field-effect transistor based on a silicon component that incorporates a gas guide channel and a large-area sensitive layer. As a result, the desired clearance or air gap between the silicon component and the substrate is created automatically. The gap is then adjusted on a controlled basis by varying the height of flip-chip bumps.
The disadvantages of known methods of fabricating such gas sensors include the risk of damaging the sensitive layer during fabrication or of the sensor material intruding into the air gap, which factors can lead to the diffusion channel being plugged, or to electrical short circuits.
FIG. 7 is a cross-sectional illustration of a prior art SGFET 1 that performs gas detection, along with the associated electrical circuitry. A gas-sensitive layer 3 is included with a gate electrode 6 in a carrier substrate 2. When a target gas diffuses through an air gap 5 onto the gas-sensitive layer 3, the result is a change in the work function and a change in potential. This potential or potential change couples to the channel region of the transistor 1 between the source and the drain, resulting in a change in the drain to source current IDS. The height of the air gap can be adjusted by the shape of the polymer bumps 8.
What is needed is a gas-sensitive field-effect transistor and a method of fabricating such a transistor to adjust the height of an air gap within the gas-sensitive field-effect transistor.